The invention relates to a motor construction that exhibits the desirable characteristics of a brush-type repulsion motor, but eliminates the conventional brushes of such a motor and their recognized disadvantages. Electronic switching means is carried on the rotating armature to short individual coils at appropriate times in a cycle of armature rotation to eliminate the need for brush and commutator elements.
The electronic switching means is in the form of power semi-conductors carried on the rotating armature. More specifically, one electronic switch circuit is provided for replacing the switch and current carrying function of one pair of oppositely disposed commutator segments or bars. Any electrical power needed to energize the electronic switching means and any related control circuitry on the armature is produced on the armature by induction from the stator field.
The control electronics on the armature includes means to sense the angular position of the armature relative to the stator in order to control the actuation of the electronic switches.
The control circuitry is operative when a coil is at a predetermined angular position, relative to the stator, to switch an appropriate electronic switch to short the ends of an associated coil together. The result of this short is essentially the same as that achieved by a pair of opposed shorted brushes.
Where the control circuitry on the rotor senses a reference point associated with the stator, the reference point or marker can be moved to different angular locations relative to the stator to change torque, speed and/or direction of the rotor. By electronically controlling the location of the reference point or marker to control torque magnitude and direction, a servomotor can be made. A significant advantage, here, is that a power amplifier is not necessary since the motor is connected directly to the AC power line.
In the repulsion design to which the invention is directed, the power electronics need only control the connections in the armature. Therefore, a large amount of mechanical power developed by the motor is controlled with power electronics that is relatively small in power handling capacity. For example, in terms of power handling capacity, the power electronics can conceivably be one-fifth to one-tenth the size of an inverter unit that would be required to drive a conventional induction motor of equivalent motor power output. The brushless repulsion motor offers other advantages.
Application of the brushless repulsion motor to replace universal series motors has the additional advantage of not having any exposed active electrical parts. This means less of an electrical shock hazard to the user of equipment such as hand tools.
Many other performance advantages and/or features accrue to the brushless repulsion motor as compared to conventional AC and DC motor designs. The electronic switch can be designed such that its opening and closing is modulated by factors other than relative position between the rotor and stator. This capability allows the design of special or tailored speed/torque curves. This capability can also provide for a more efficient conversion of energy on start-up which lowers start-up currents and thereby eliminates the need to employ separate electronic "reduced voltage" start-up controls found in many industrial applications and which are added items of expense.
With regard to the use of the brushless repulsion motor as a servomotor, its performance characteristics are exceptionally good with respect to dynamic response. With the brushless repulsion motor, only a part of the total amount of electrical power is contained in the rotor/armature, therefore the electronics need not process all of the electrical power. By contrast, in a permanent magnet DC servomotor known in the art, all of the electrical power must enter and be processed through the armature by an external electronic power control or amplifier. The electrical time constant of the armature is a dominant factor that limits the dynamic response. In modern high performance AC servos an induction motor is used with a control algorithm referred to as "flux vector control" or "field oriented control". All of the electrical power is supplied by an external power amplifier or inverter and the control calculates the relative position of the stator field to the rotor field to provide optimum control response. Here again, the limitations involve the rotor electrical time constants and the stator electrical time constants. Also, the torque in the brushless repulsion motor, is developed by a change in relative position of the rotor field and stator field. Changing this position requires only to change an external reference marker position and this can be done in many ways that are extremely fast. For example, photodiodes and phototransistors can be used to change the position of the reference marker. The electrical time constant of the rotor/armature is also a diminished factor because not all of the windings are switched at the same time and this creates, in effect, a reduced time constant.
This brushless repulsion motor with reference markers at fixed positions around the stator has a speed dependent upon the applied torque to the rotating armature. Consequently, as the torque is decreased, the speed increases. In a like manner, as the torque increases, the speed decreases. Thus, this highly beneficial brushless repulsion motor is deficient in the area of speed control. The armature speed can not be controlled at a desired operating speed without external circuitry.